Answer
$u_1 = 4~u_2$
Work Step by Step
We can write an expression for the drift velocity:
$v_d = \frac{I}{n~q~A} = \frac{I}{n~q~\pi~r^2}$
$I$ is the current
$n$ is the number of electrons per unit of volume
$q$ is the charge of one electron
$A$ is the cross-sectional area
$r$ is the cross-sectional radius
We can write an expression for the drift speed $u_1$:
$u_1 = \frac{I}{n~q~\pi~r_1^2}$
We can find an expression for the drift speed $u_2$:
$u_2 = \frac{I}{n~q~\pi~r_2^2}$
$u_2 = \frac{I}{n~q~\pi~(2r_1)^2}$
$u_2 = \frac{1}{4} \times \frac{I}{n~q~\pi~r_1^2}$
$u_2 = \frac{1}{4} \times u_1$
$u_1 = 4~u_2$
